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The striking and rare Haleakalā silversword, found only on the
high volcanic slopes of Maui, is on the decline, scientists
report today (Jan. 15) in the journal Global Change
Biology.

First, the plant was nearly killed off by cows and collectors
starting in the 1880s, then conservationists made it a success
story after the 1930s. Now climate change is bringing about a new
collapse.

The culprit is
shifting weather patterns, which have made the plant's
environment too dry and warm for new seedlings to survive. Older
plants are dying, too, said study co-author Paul Krushelnycky, a
biologist at the University of Hawaii at Manoa.

Plummeting numbers

The numbers tell the tale: From a low of 4,000 in the 1920s to a
high of 61,000 in 1991, the plant population is now dropping. A
sample census counted 28,492 in 2010 — but not all of them were
alive. "It wasn't obvious at first, because when they die they
remain in place for many years," Krushelnycky told
OurAmazingPlanet.

The 2010 survey atop Haleakalā volcano on the
island of Maui revealed less than half of plants (47 percent)
at lower elevations around 7,100 feet (2,185 meters) were alive,
indicating a substantial decline since the 1990s.

Baby plants are also struggling to grow in the drier conditions.
Even with population booms
during wet years, the seedlings die within two to three
years, the study found.

"If these climate trends continue, it doesn't look good for this
subspecies," Krushelnycky said.

Lack of rain

The Haleakalā silversword is iconic in Hawaii, which has
more endangered species than any other state. With a
ball-shaped base and hairy, silvery leaves, the plants are one of
30 species in the silversword alliance. The alliance evolved from
a small, daisy-like plant called the tarweed that arrived in
Hawaii from California about 5 million years ago, Krushelnycky
said. "The silversword is one of the more extreme forms, but it
can grow next to one of its relatives, like a green-leafed shrub,
and actually hybridize," he said.

Living for 40 to 50 years on thin, poorly developed volcanic soil
in high winds and temperatures that regularly drop below
freezing, the silversword is literally a textbook example of
biological adaptation.

The Haleakalā silversword flowers only once, usually in summer,
sending a spike up to 6 feet (2 meters) tall into the sky with as
many as 600 blooms. Then the plant dies.

A combination of climate changes is stressing the silverswords,
the researchers said. Local temperatures are warming, but the
biggest factor affecting the plant's growth rate is a dropoff in
annual rainfall, Krushelnycky said.

Rainfall in Hawaii is driven by trade wind patterns, and there
are
fewer days with moisture-laden winds than 40 years ago.
Silverswords also get moisture from breaks in the island's
inversion layer, which traps moist air below its cooler, drier
air. "We're getting fewer interruptions of that inversion layer,
and fewer moisture events are getting into their habitat,"
Krushelnycky said.